The present disclosure relates to a multiple parameter sensor-transmitter/receiver unit which may be installed on or removed from an energized electric power line, such as an overhead power line. With the advent of Smart-Grid applications for electric power systems, there is an ever increasing need for a device that measures electric, mechanical, and environmental parameters of the power line.
In order to address the increasing need for monitoring power lines, devices have been developed that attach directly to the power line. These devices generally require a power source, such as batteries or solar panels. When utilizing batteries, regular maintenance must be performed to replace the batteries, which can become costly. When solar panels are used, the device may only be power during sunny weather conditions and during daylight hours. Therefore, there is a need for a device which is low maintenance and can be constantly powered independent of weather conditions.
The function of a voltage transformer is to produce a low secondary voltage output, typically 120 volts, which is representative of the high primary voltage of the electric power distribution line or transmission line. The low secondary voltage output is proportional to the turns ratio of the transformer and would be in phase with or in phase opposition to the primary voltage depending upon its connection to the primary phase wires. However, the secondary voltage could only be in phase with the primary voltage for an ideal transformer which has no leakage impedance, no excitation current, and no losses. But, actual voltage transformers require excitation current to magnetize the iron core which is supplied by the primary lines through the leakage impedance of the primary. Also, the load current in the transformer windings causes a voltage drop in the leakage impedance of the primary and secondary windings. The load current and excitation current cause a voltage drop which results in a turns ratio error and a phase angle shift other than a 180 degree phase shift between the primary voltage and the secondary voltage output. Therefore, a turns ratio correction factor and a phase angle correction factor must be applied to obtain the true ratio and the true phase angle between the measured voltage and the measured current on the primary, or the power factor.
Other methods of measuring the voltage have included (1) high voltage divider circuits, (2) voltage based on measurement of the electric field outside the conductor at some point, and (3) voltage based on measurement of the electric field using optics. The high voltage divider method normally involves a high value resistor or resistors being attached to one line or phase conductor at one end and the other end of this high value resistor or resistors being connected in series with a low value measuring resistor with its other end being attached to earth ground at zero voltage. The voltage measurement is then a measurement of the low value current flow from the high voltage line conductor to ground through these series connected resistors and the resultant voltage drop across the measurement resistor. This voltage drop can easily be measured with instruments at ground potential. The problems with this method are the resistors are dissipative and can result in overheating and it cannot be used to measure phase to phase voltages because of the high voltage that appears on the measurement resistor prevents the use of measuring instruments normally located at ground potential. The series connected resistors can be replaced with series connected capacitors which are non-dissipative, but then a phase shift occurs in the measured voltage.
Voltage is defined as the integral of the electric field between two points, the one point being the line or phase conductor itself and the other point being in space around this conductor. The problem with this method is the electric field in space around one phase conductor can be significantly affected by the presence of the other phase conductors of a three phase system and various structures which support these phase conductors may be steel towers or poles which are grounded, or wood poles which have ground leads connected to lightning arresters or other equipment grounds. Therefore, the measurements of the electric field and thus the voltages are very inaccurate. The use of optics has been employed, but their use thus far has resulted in flashovers across the fiber optic lead wires in high voltage transmission lines applications and the cost of such devices have been excessive, especially in high voltage measurement applications.